Problem:
Let f1β(x)=1βxβ, and for integers nβ₯2, let fnβ(x)=fnβ1β(n2βxβ). If N is the largest value of n for which the domain of fnβ is nonempty, the domain of fNβ is {c}. What is N+c ?
Answer Choices:
A. β226
B. β144
C. β20
D. 20
E. 144 Solution:
Because f2β(x)=1β4βxββ,f2β(x) is defined if and only if 0β€4βxββ€1, so the domain of f2β is the interval [3,4]. Similarly, the\
domain of f3β is the solution set of the inequality 3β€9βxββ€4, which is the interval [β7,0], and the domain of f4β is the solution set of the inequality β7β€16βxββ€0, which is {16}. The domain of f5β is the solution set of the equation 25βxβ=16, which is {β231}, and because the equation 36βxβ=β231 has no real solutions, the domain of f6β is empty. Therefore N+c=5+(β231)=β226β .